Automotive disc brakes consist of a rotor and a caliper. The rotor rotates with the wheel and axle. The caliper does not rotate and holds the brake pads. The caliper is mounted over and on either side of the rotor. When the brake is actuated, the caliper forces the pads against the rotor to dissipate energy and stop the car. The rotor, subject to high stress and heating, has traditionally been made of cast iron, which is a relatively heavy material. There has been considerable interest in developing rotors made out of relatively light weight materials such as aluminum and magnesium, in an effort to reduce vehicle weight and increase fuel economy. Such relatively lightweight metal rotors are coated with a hard surface layer for proper friction as well as wear and resistance. This hard layer can be applied by several processes, for example by thermal spraying a metal powder or wire onto the surface of the rotor.